Receivers for phase comparison radio navigation systems

ABSTRACT

In a receiver for a phase comparison radio navigation system, signals of the fifth, sixth, eighth and ninth harmonics of a fundamental are received from different spaced transmitting stations and compared in phase in pairs at their least common multiple frequencies and in which a phase comparison is made between signals from the same pairs of stations at the fundamental frequency to obtain coarser but less ambiguous positional information, the fundamental frequency comparisons are effected digitally measuring the time intervals corresponding to the phase relationships. These time intervals are measured using divided down frequencies from a single clock source, the clock frequency division factors being different for the different pairs of stations so that the counts are proportional to the least common multiple frequencies.

United States Patent Hughes Dec. 5, 1972 David Geoffrey Hughes, London, England Decca Limited, London, England Nov. 9, 1970 Inventor:

Int. Cl ..G0ls 1/30 Field of Search ..343/105 R [56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,016,981 1/1966 Great Britain ..343/105 H Primary Examiner-Benjamin A. Borchelt Assistant Examiner-Richard E. Berger Attorney-Mawhinney & Mawhinney [5 7] ABSTRACT In a receiver for a phase comparison radio navigation system, signals of the fifth, sixth, eighth and ninth harmonies of a fundamental are received from different spaced transmitting stations and compared in phase in pairs at their least common multiple frequencies and in which a phase comparison is made between signals from the same pairs of stations at the fundamental frequency to obtain coarser but less ambiguous positional information, the fundamental frequency comparisons are effected digitally measuring the time intervals corresponding to the phase relationships. These time intervals are measured using divided down frequencies from a single clock source, the clock frequency division factors being different for the different pairs of stations so that the counts are proportional to the least common multiple frequencies.

to cu STURE REGISTER.

DISPLAY l REGISTER,

SHEET 1 BF 5 FIGUREB.

FIGURES.

FIBUREL.

PATENTED DEC 5 I972 SHEET 2 BF 5 RECEIVERS FOR PHASE COMPARISON RADIO NAVIGATION SYSTEMS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to receivers for phase comparison radio navigation systems.

2. Prior Art The invention is concerned more particularly with receivers for the system, known as the Decca (Registcred Trade Mark) Navigator system in which a master station normally radiates radio frequency signals of a frequency of and three slave stations, known as the red, green and purple slaves, normally radiate signals of frequencies 8f, 9f and 5 f respectively, 7

f being the fundamental frequency of the chain. All the radiated signals are held in a fixed phase relation. In the receiver the signals from the master station are separately compared in phase with signals from each of the slave stations. These comparisons are effected at the least common multiple frequency for each pair and thus may be effected at the frequency 24f for red, 18f for green and 30f for purple. Any due phase comparison provides information in the form of a hyperbolic position line in a set of position lines, referred to hereinafter as a pattern. There are thus red, green and purple patterns provided by a complete system with three slave stations.

In practice, receivers may have to be used with any of a number of different chains. The different chains use slightly different values for f; in the receiver, the various received signals, (any one of which may be expressed as a frequency nf) are heterodyned with signals of frequency n A where A f F. F is a fixed frequency. By using the lower sidebands from the mixers, the output signals are 6F, 8F, 9F and SP for the master and the red, green and purple slaves. The comparisons may then be effected at 24F, 18F and 30F. In the following description therefore, the more general case will be considered where the comparisons are at these frequencies 24F, 18F and 30F but this does not exclude the special case of a non-heterodyne receiver where F f. The present invention is concerned with the receiver and, except where the content clearly implies otherwise, all references to the fundamental frequency will refer to the effective fundamental frequency employed in the receiver, i.e., F in the more generalized case.

The accurate positional information obtained by the comparisons at least common multiple frequencies is however ambiguous because, in practice, many complete cycles of phase change occur in traversing the operational area of the system. Further signals are therefore radiated by the transmitting stations to enable a phase comparison to be made in the receiver at a much lower frequency of IF. A number of different ways of radiating further signals for this purpose have been used. For example, for short periods, the radiations from all the stations may be interrupted, and all the frequencies radiated in fixed phase relation from provide 1F signals at the receiver which may be compared with one another, for example, by comparing each in phase with a 1F signals from an oscillator in the receiver locked to one of the received signals, usually the 6P signals from the master station.

It will be seen that a phase comparison at a frequency 1F gives one cycle of phase change covering 24 complete cycles or lanes of the 24F red pattern comparison. For the green pattern however a phase comparison at 1F covers only 18 cycles or lanes, of the 18F comparison. For the purple pattern, a cycle at 1F covers 30 cycles or lanes of the 30F comparison. These 1F phase comparisons are for the purpose of identifying the lanes in the various different patterns and the present invention is concerned with the display of the IF information so that the lane numbers for the various patterns may be directly identifiable.

SUMMARY OF THE INVENTION According to this invention, in a receiver for a phase comparison radio navigation system of the kind in which radio frequency signals of three or more different but harmonically related frequencies received from different spaced transmitting stations are compared in phase in pairs at their least common multiple frequencies to provide positional information with respect to fine patterns and in which coarser positional information to identify the lanes of the fine patterns is obtained by making phase comparisons at the fundamental frequency for each pair of stations, is characterized in that the fundamental frequency comparisons are effected digitally using a counter to count pulses from a clock source during a time interval representative of the phase difference between the signals to be compared, the clock pulse source including an adjustable frequency divider controlled in accordance with sequential comparison of signals from different pairs of stations to display counts related to the phase difference, the respective counts, by adjustment of the divider, being proportional to the respective least common multiple frequencies used for the fine pattern of each pair.

In the simplest form, each phase difference between the fundamental frequency signals may be measured by using the two signals to be compared in phase to gate a clock output of the appropriate frequency. Commonly however the least common multiple frequencies are relatively small numbers, e.g., 24, 18 and 30 and to avoid the more complex division using reciprocals of these numbers, it may be more convenient to have the phase difference measured as a time difference, into a first store using a fixed clock frequency and then to count a fixed clock frequency (which need not be the same as the previously mentioned fixed clock frequency) into a display store for a period determined by the time required to count the number in the first store using a divided clock frequency.

The fine positional information is preferably indicated using a separate indicator for each pattern, each indicator having a pointer which traverses a circular scale, one revolution corresponding to one cycle of phase change at the least common multiple frequency of the signals being compared; each indicator for the fine positional information conveniently has a mechanical counter driven through reduction gearing to indicate the number of complete revolutions of said pointer. With such an arrangement, said adjustable divider is set for each comparison at the fundamental frequency so that the digital counter indicates in decimal digits the correct lane count determined by the phase comparison at the fundamental frequency for setting the appropriate mechanical counter. It will be seen that the mechanical counters provide continuous lane identification information provided they are initially set to correct value. The multipulse signals provide information from which the lane identification is determined. This information is available however periodically in sequence. With the arrangement described above it is displayed on a single digital counter. The operator can read this counter and use the information to set the mechanical counters. The digital display enables the setting of the mechanical counters to. be checked if required but these counters then provide a continuous display of lane identification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of a receiver and display unit for a phase comparison radio navigation system;

FIG. 2 is a schematic diagram of a radio frequency/intermediate frequency unit forming part of the receiver of FIG. 1 and including also certain controls;

FIG. 3 is a schematic diagram of a locked oscillator unit of the receiver of FIG. 1 showing also certain display indicators;

FIG. 4 is a schematic diagram of a heterodyne and locked oscillator unit of the receiver of FIG. 1, showing also certain further controls;

FIG. 5 is a schematic diagram of a lane identification and timer unit of the receiver of FIG. 1 together with a further display unit; and

FIG. 6 is a diagram showing how FIGS. 2, 3 4 and 5 fit together to form the complete receiver display system.

DESCRIPTION OF THE PREFERRED EMBODIMENT The receiver and display unit show-n in the drawings is for use with fixed transmitting stations of the Decca (Registered Trade Mark) system. Each chain of transmitters comprises a master station and usually three slave stations, known as the red, green and purple slaves. The master station normally radiates continuous signals of a frequency 6f where f is a fundamental frequency of the order of 14 kHz. The red, green and purple slaves normally radiate on frequencies 8f, 9f and 5 f. All the radiated signals are locked in phase. Periodically the transmissions from all the stations are interrupted and a short duration signal of all the frequencies 5f, 6f, 8f and 9f in fixed phase relation is radiated from one station. This signal is referred to as the multi-pulse signal. The multipulse signal is radiated in sequence from the master station and the red, green and purple slaves. Each multipulse transmission is preceded by a break of 0.1 seconds in the normal master (6f) transmission.

The normal transmissions are utilized in a mobile receiver by separately comparing the phase of each of the received slave signals (8f, 9f and 5f) with the received master of signals. The comparisons are effectively at the lowest common multiple frequencies. The phase angle is indicated to provide fine positional information with respect to three sets of hyperbolic position lines. This final positional information corresponds to the phase angle within a cycle but is ambiguous in that each hyperbolic pattern covers many complete cycles. The various different cycles of the pattern are referred to as lanes. The multi-pulse signals are used for lane identification. In this receiver the effective comparison frequency employed is the fundamental frequency. A phase measurement at this frequency gives a coarser indication which serves to identify a lane within a zone, a zone being 24 lanes for the red-pattern, 18 lanes for the green pattern and 30 lanes for the purple pattern.

The receiver can be utilized with any of a number of different chains. The different chains have fundamental frequencies which differ by small amounts. This is achieved, as will be more fully explained later by superheterodyne operation to convert the received signals of 5f, 6f, 8fand 9fto 5F, 6F, 8F and 9F by mixing with the appropriate harmonics of a hererodyne frequency A where A f F, using the lower side bands of the mixer outputs. Thus the normal phase comparison is effected in the circuitry at the frequencies 24F for red, 18F for green and 30F for purple whilst the lane identification has a comparison frequency of 1F, although the effective frequencies are 24f, 1 8f and 30ffor the normal patterns and If for lane identification.

Referring now to FIG. 1, the receiver and display unit illustrated is particularly for marine use and comprises a base 10 with a trunnion mounted display unit 11 containing all the circuitry which, with the exception of a cold-cathode number tube display, is all solidstate utilizing printed circuit boards. The power supplies are provided by a power supply unit (not shown) fitting onto the bottom of the assembly of FIG. 1, this power supply unit being interchangeable so that an appropriate unit may'be fitted according as to whether the external supply is AC. or DC. and to its voltage and frequency.

In the upper part of the unit 11 are the red, green and purple fractional lane indicators 12, 13 and 14 (known as decometers), each comprising a pointer rotatable over a dial. Associated with each of the fractional lane indicators are integrating lane counters in the form of rotating discs. It will be seen that each fractional lane indicator has a lane indicator 15 and a zone indicator 16. The lane indication from the multiple signals is shown on a three digit cold cathode number tube display 17.

The panel of the upper unit also includes a lock lamp 18 to be described later, a lane identification zero push button 19 and a dimmer control 20 for the display 17.

In the lower part of the assembly, a hinged cover 21 is provided for covering certain controls, primarily for setting up, these controls being a pair of multiposition switches 22, 23 for chain selection, zero setting controls 24, 25 and 26 for the three decometers and a function switch 27.

Referring now to FIGS. 2 to 5, the main units, each formed by a single circuit board and indicated by dashed line boxes, are an R.F/I.F unit 30 (shown in FIG. 2), a heterodyne and reference oscillator unit 32 (shown in FIG. 4), a locked oscillator unit 33 (shown in FIG. 3) and a lane identification and timer 34 (shown in FIG. 5) with which is associated a smaller board 35 (also shown in FIG. 5) for the display 17. FIGS. 2 to 5 fit together as shown in FIG. 6.

An aerial (FIG. 2) is connected via a buffer/limiter 41 and gate 42 to amplifiers 43, 44, 45 and 46 tuned respectively to 5f, 8f, 9fand 6fbut of sufficient bandwidth to amplify signals from any selected chain. The outputs from these amplifiers are fed respectively to mixers 47, 48, 49 and 50 where they are mixed with the appropriate harmonics of the A signals from the unit 32. For this purpose, the A signals on a lead 51 from unit 32 are fed to multipliers 52, 53, 54 and 55 to provide the appropriate harmonics. Associated with the multipliers 52, 53 and 54 respectively are the zero setting controls 26, 24 and 25 which, although shown within the box 30 of FIG. 2, are located on the lower panel as shown in FIG. 1. The required sidebands of SF, SF, 9F and 6F from the mixers 47, 48, 49 and 50 are selected by band pass filters 56, 57, 58 and 59 and the filter outputs are amplified by amplifiers 60, 61, 62 and 63 and are passed to phase discriminators 64, 65, 66 and 67 where the received signals are respectively compared in phase with signals of the same frequencies from oscillators 68, 69, 70 and 71 in the unit 33 (FIG. 4). The phase discriminators 64, 65, 66 and 67 are of the sampling type, the outputs of the oscillators 68, 69, 70 71 being fed to pulse producing circuits 72, 73, 74, 75 respectively to provide the short duration sampling pulses for applying to the phase discriminators. The phase discriminators are sine output discriminators, there being zero outputs when the oscillator outputs are in phase with the received signals; the discriminator outputs are thus D.C. voltages and they are applied to integrator/reactors 76, 77, 78 and 79 (FIG. 4) to control the respective oscillator frequencies. In two positions, LOCK 1 and LOCK 2 of the function switch 27 (FIG. 2), fast lock switching is applied via a lead 28 to the integrator/reactors 76, 77, 78 and 79. These integrator/reactors serve'to control the frequencies of the respective oscillators so as to maintain a phase lock between each oscillator output and the respective input from the amplifiers 60, 61, 62 and 63. A further description of the oscillator control loop is given in the specification ofU. S. Pat. No. 3,603,893 entitled Phase Locked Oscillators.

The pulse outputs from the oscillators 68, 69 and 70 (at frequencies SF, SF and 9F respectively) are fed to frequency multipliers 80, 81 and 82 to give signals at frequencies 30F, 24F and 18F respectively. Considering' the 30F signal from the multiplier 80, this signal is fed directly to a sine output phase discriminator 83 and is also fed via a quadrature phase shifter 84 to a cosine phase discriminator 85. These are sampling type phase discriminators, the sampling pulses being at 6F from the oscillator 71 via the pulse shaper 75 and an amplifier 87. The sine and cosine outputs from the phase discriminators 83, 85 are applied via d.c. amplifiers 88, 89 to the purple decometer 14. Similarly the red decometer 12 is driven by phase discriminators 90, 91 and d.c. amplifiers 92, 93 to indicate the phase relation between the SF and 6F signals and the green decometer 13 is driven by phase discriminators 94, 95 and d.c. amplifiers 96, 97 to indicate the phase relation between the 9F and 6F signals. It will be noted that, by using locked oscillators to drive the decometers, the locked oscillators perform effectively as very narrow band filters, providing continuous noise-free drive to the decometers and bridging the interruptions in received signals during the short signalling breaks and during the multipulse transmissions.

Each of the decometers 12, 13 and 14 has a rotor carrying a pointer which traverses a circular scale. The angular position of the rotor is determined by the relative magnitudes and polarities of d.c. signals applied to orthogonal coils of the decometer. These d.c. signals are obtained from the d.c. amplifiers 88, 89 for the purple decometer and the corresponding amplifiers for the other decometers and hence correspond to the sine and cosine of the phase angle between the master and respective slave signals as received at the receiver. The angular position of the rotors thus corresponding to the phase angle. The changes of phase angle are mechanically integrated and complete cycles of phase change are indicated by the associated lane indicator 15 which is driven through a reduction gear from the decometer rotor. The zone indicator 16 for each decometer is likewise driven through further reduction gearing from the lane indicator.

The decometers may be adjusted manually by means of manual control knobs 180. Operation of these controls enables the lane and zone indicators to be reset but the rotor of each phase angle indicator will take up a position corresponding to the measured phase angle. These knobs 180 therefore enable the lane and zone indicators 15, 16 to be reset without affecting the accura' cy of the positional information from the phase angle measurement within a cycle.

The 6F pulse output from the pulse shaper 75 is used not only as the sampling pulse for the phase discriminators driving the three decometers 12, 13 and 14 and in the phase lock control loop for the GP oscillator 71 but is fed also to a frequency divider 98 to give 1F pulses for purposes to be described later and is also fed via a quadrature phase shifter 99 to a cosine phase discriminator 100 (FIG. 2). In this discriminator 100, the 6F signals from amplifier 63 are sampled. This discriminator 100 thus gives its maximum output so long as the normal 6F signals from the master station are being received and the 6F oscillator 71 in the receiver is locked in phase to the GP signals from amplifier 63.

The output from the discriminator 100 is fed via a lead 101 to an amplifier in the heterodyne and reference oscillators 32 (FIG. 4). The output from the amplifier 110 is fed to the lock lamp 18 which will thus blink if the oscillator 71 is not properly phase locked. If the oscillator 71 is locked, the lamp 18 will remain continuously lit except during the multipulses and the breaks in the master transmissions immediately before the multipulses. The amplifier 110 also provides the input to a break detector" 111 to detect the restoration of the (H second after the break and so give an output signal which indicates the start of each multipulse.

The heterodyne and reference oscillator 32 contains a frequency synthesizer providing the heterodyne frequency A and also a reference oscillator for referencing the receiver.

The frequency synthesizer comprises a 436.907 kI-lz crystal oscillator feeding a divider train 121 and a matrix 122 for combining the divider train outputs, the matrix 122 being controlled by the aforementioned chain selector switches 22, 23. The synthesized frequency output from the matrix is used to control the frequency of a heterodyne oscillator 123 having a phase lock control loop consisting of a sawtooth output discriminator 124 giving a d.c. output voltage applied to a reactor 125 controlling the oscillator frequency. The A frequency output from the oscillator 123 is used for frequency changing in the aforementioned mixers 47, 48, 49 and 50. The reference oscillator for referencing the receiver is an 8F oscillator 130. This oscillator is phase locked to the IF pulses from the aforementioned divider 98 (FIG. 3), these pulses being applied via a lead 102 to a linear phase discriminator 131 (FIG. 4) having a 1F sampling pulses from a divide-be-eight divider 132 fed from the output of oscillator 130 and the dc. output voltage of discriminator 131 being applied to a reactor 133 to control the frequency of oscillator 130. The IF output from the divider 132 is converted into 1f pulse signals by means of a mixer 134, receiving A frequency signals from oscillator 123, and a pulse shaper 135. The 1f pulses from the shaper 135 can be applied to the receiver input channels 43, 44,45 and 46 (FIG. 2) via a gate 136. This gate 136 and the aforementioned gate 42 are controlled by the function switch 27; normally only the received signals are applied to the receiver input channels but, when the function switch 27 is in the Reference position, these signals are disconnected from the receiver channels by closing gate 42 and the 1f pulses are applied by opening gate 136. These pulses form a series of harmonics of the frequency 1f in a fixed multiple phase relation and the decometers 12, 13 and 14 are set to zero using the zero setting controls 24, 25 and 26. During referencing, the 6F oscillator 71 is not locked to received signals but the stability of this oscillator is such as to maintain the frequency of the reference oscillator 130 very closely during the referencing operation.

The lane identification is obtained by measuring the phase lag of an effective 1 f signal from each slave station compared with a l f signal from the master station. These 1f signals from the slaves are provided by the multipulse transmissions and are used after frequency changing to 1F. The master oscillator 71 provides the master 1F reference.

The multipulse signals for lane identification purposes are obtained in the receiver by combining the outputs at SF, 8F, 9F and 6F from the amplifiers 60, 61, 62 and 63 in a multipulse circuit 140 (FIG. 5). The phase relationship of the radiated 5f, 8f, 9f and 6f signals in each multipulse transmission in such that they combine to give a peak pulse at a frequency 1f. Thus the SF, 8F. 9F and 6F signals from the amplifiers 60, 61, 62 and 63 give a peak pulse at a frequency 1F and this is used to set a bistable 141 which is reset by the IF output from the frequency divider 98 (FIG. 3) on lead 139. The bistable 141 (FIG. 5) controls a gate 142. The multipulse signal has a duration of a fraction of a second but contains many cycles at 1F. The gate 142 is cocked by a short duration lane identification read time pulse, referred to as the second L.I pulse obtained on a lead 151 from switching logic 147 to be described later. The gate 142 is cocked only for the duration of this second L.I pulse and serves only to pass pulses durlng one time period starting on a 1F multipulse signal and ended by the next 1F pulse from the divider 98. The gate 142 gates pulses from a 300F oscillator 143 into a store 144 via an OR gate 145. The multipulse circuit 140. from the received multipulse 1F signals, thus effectively provides one pulse synchronized with the effective 1F from the multipulse. This pulse opens the gate 142 and the next 1F signal from the divider 98 closes the gate. The multipulse signal will therefore produce a stored count in store 144 proportional to the multipulse-to-master delay, i.e. proportional to the complement of the Master:Multipulse zone pattern phase angle in 1/ 300th zone units. The total capacity of the store 144 corresponds to one complete cycle at 1F, i.e. a count of 300 at 30oF. The required lane identification is obtained by determining the residual capacity in the store 144, this being done by filling the store until it overflows and determining the quantity required in l/300th zone units for purple but in l/240th zone units for red and 1/180th zone units for green. To do this, the store 144 is filled from the 300F oscillator 143 via a divider 146 which divides by a factor of 10 or 8. or 6 under the control of lane identification switching logic 147, to be described later, the division factor being 10 for purple, 8 for red and 6 for green. The divider output is fed to the store 144 via the OR gate 145. Simultaneously, the 300F signals from the oscillator 143 are fed via a 10 to 1 divider 148 and a gate 149 into a display register 150. The gate 149 is a two input AND gate having, as a second input a pulse signal derived from a pulse generator 152 which is started by a pulse, known as the fourth LI pulse from the logic unit 147 on lead 153 and terminated by a stop pulse from the register 144 when the latter is full. The fourth LI pulse is a timing pulse slightly delayed after the second LI pulse. Thus, considering the red lane identification, the divider 146 is set to divide by 8 and so the ratio of the number fed into the display register to the number fed into the store 144 is 8 to 10. Similarly for the green lane identification, the ratio is 6 to 10. Thus, although the pulses fed into the store 144 are in units of l/300th of a zone, those fed into the display register are in units of l/240th of a zone for red, 1 th of a zone for green and l/300th of a zone for purple; in other words they now correspond to one tenth of a lane for each of the patterns.

The display register 150 has to be filled only during the time taken for the store register 144 to be filled after the initial count of the complement of the master to multipulse period. To do this the display register has to be reset for the start of the required period and the gate 149 closed when the store register 144 is completely full. This closing of the gate 149 is effected by an output signal from register 144 when this store register is full. The display register is reset for each multipulse by a first LI pulse from the switching logic 147. In the Decca (Registered Trade Mark) Navigator system, to avoid any possibility of confusion between readings on different patterns, the 24 lanes in a red zone are numbered from 0 to 23 and thus a red lane identification (to a tenth of a lane) is expressed as a number between 0 and 23.9. The 18 lanes in a green zone are numbered from 30 to 47 and so the green lane identification is expressed as a number between 30 and 47.9. The 30 lanes in a purple zone are numbered from 50 to 79 and so the purple lane identification is expressed as a number between 50 and 79.9. For these reasons, the display register 150 is set to zero at the beginning of the red lane identification count into that register, to 30 at the beginning of the green count into that register and to 56 at the beginning of the purple count into that register. The switching logic 147, in

resetting the display register, therefore has appropriately to set the most significant decimal digit of the display register which is a binary coded decimal counter.

The lane identification switching logic 147 provides output gating signals referred to as the first LI pulse, on four leads 160, 161, 162 and 163 indicating the start of the master, red, green and purple multipulse transmissions respectively, these signals being used for setting the divider 146 to divide by the appropriate factor and to reset the display register 150 before the start of each count into that register. For this purpose, the switching logic 147 makes use of the break detector 111, and time division of 26.6 Hz and 13.3 Hz signals from the divider train 121 to provide a cycle of approximately second period during which an output appears successively on the leads 160, 161, 162 and 163 at 2.5 second intervals. The master multipulse is the first of each sequence of four pulses and is identified thereby in the switching logic. The first LI pulse for each multipulse which is used for setting the store register 144 via lead 158 is timed to occur in the latter half of the multipulse period and is timed to occur about 0.3 to 0.4 of a second after the break in the master transmission. The second, third and fourth Ll pulses follow the first LI pulse in each multipulse with small sequential delays appropriate for their respective logical operations.

The master multipulse LI signal on lead 160 is applied, by means of an OR gate 164, to the divider 146 and display register 150 in the same way as the red multipulse signal and is utilized for zeroing the lane identification outputs. In the receiver described, the frequency divider 98 dividing the 6F oscillator output down to IP is not notched," that is to say, the IF outout is not locked to any specific one of the six cycles of the 6F signal. There is therefore a sixfold possible ambiguity in the phase relation of the IF output from divider 98 with respect to the IF multipulse from the master station fed to the multipulse circuit 144. Acorrection is therefore put in the store register 144 to bring the lane identification reading on display register 150 to zero on the master multipulse transmission and this same correction is used for each of the other multipulse transmissions thereby correcting for the absence of notching on the divider 98 and also correcting for any other phase errors which might arise between the multipulse take-off and the master oscillator (6F oscillator 71) locking circuit. This correction is effected using a phase" store 170 and a bistable 171. The bistable 171 can be set by depressing the lane identification zero push button 19 and it then provides a signal to a gate 172, which gate, when opened admits 30F pulses from divider 148 into the phase store 170. The bistable 171 is reset the next time the store register 144 is complete ly filled. The gate 172 is a four input AND gate and has an input on lead 156 from logic unit 147 so that the gate 172 is open only during master multipulse transmissions and not during slave multipulse transmissions. The fourth input to gate 172 is a third LI pulse on lead 157 from logic unit 147 so that the gate is only opened for the required appropriate time duration during the master multipulse transmission. The number of 30F pulses fed into the phase store 170 via the gate 172 is thus the same as the number fed into the display register 150. The reason for the four inputs to gate 172 is to ensure that the phase error reading is injected into store 170 at the third LI pulse only when the button 19 is pressed and only on a master multipulse transmission. The pulses from gate 172 are also fed via lead 173 into gate and thus into the store register 144. When this register is full, it spills over and bistable 171 is reset. Thus the phase store 170 has the required correction and register 144 is in the zero state. Gate 181 ensures that the bistable 171 is reset only between the third and fourth LI pulses. This prevents bistable 171 being reset too early if the zero button 19 is pressed for example, immediately after the previous master lane identification.

The number thus put in the phase store 170 is then applied as an initial input to the store register 144 each time the latter is reset by the first LI pulse. Functionally, it is required that the phase store 170 should contain the phase reading on the master multipulse transmissions and thus the button 19 should be depressed before a master multipulse transmission. In practice however, the button can be depressed at any time and the next master multipulse reading on the display register is 00.0 or 23.9. The required correction is thus set into the phase store 170. The count remains in the store and is used for correction of each subsequent count in the store register 144.

The output on the display register 150 is visually indicated on the three tubes of the number tube display 17 (FIG. 1). In FIG. 5, these three tubes are shown at 174, 175 and 176, having associated binary-codeddecimal to decimal converters 177, 178 and 179 respectively. The master red, green and purple indications are displayed in sequence in that order, at 2. 5 second intervals with a longer interval before the cycle repeats in its 20 second period. Because the numerical values of the green readings must be between 30 and 47.9 and the purple readings between 50 and 79.9, the various readings can readily be distinguished. However, to avoid any possible confusion between the master reading (which is 00.0 or 23.9 after the correction has been set in) and the red reading, the display is made to blink during the master indication, a suitable control signal being obtained from the switching logic 147. The displayed lane identification information is used to set in the lane readings on the indicators 15 of the decometers 12, 13, 14 using the manual control knob 180 on the respective decometers.

lclaim:

1. In a receiver for a phase comparison radio navigation system of the kind in which radio frequency signals of three or more different but harmonically related frequencies received from different spaced transmitting stations are compared in phase in pairs at their least common multiple frequencies to provide positiOnal information with respect to fine patterns and in which coarser positional information to identify the lanes of the fine pattern is obtained by making phase comparisons at the fundamental frequency for each pair of stations; the improvement comprising the provision of digital phase comparison means for the fundamental frequency comparisons, which phase comparison means comprise a clock source, an adjustable divider dividing down the clock pulse frequency, a counter to count pulses from a said divider during a time interval representative of the phase difference between the signals to be compared, and means controlling the divider in accordance with the sequential comparison of signals from different pairs of stations whereby the counter displays counts related to the phase difference, the respective counts, by the controlled adjustment of the divider, being proportional to the respective least common multiple frequencies used for the fine pattern of each pair.

2. A receiver as claimed in claim 1 wherein each phase difference between the fundamental frequency signals is measured by using the two signals to be compared in phase to gate a clock output at the appropriate frequency.

3. A receiver as claimed in claim 1 wherein each phase difference between the fundamental frequency signals is measured by measuring the corresponding time difference into a first store using a fixed clock frequency and then counting a fixed clock frequency (not necessarily the same as the aforementioned fixed clock frequency) into a display store from a period determined by the time required to count using a divided clock frequency.

4. A receiver as claimed in claim 1 wherein said clock pulse source has a frequency which is a multiple of the fundamental frequency, and wherein said adjustable divider for dividing down the frequency of the clock pulse source, said division being by selectable factors in the same ratio to one another as the aforesaid least common multiple frequencies, and wherein there are provided a store register and a display register said store register having a capacity equal to the number of clock pulses in one cycle of the fundamental frequency, means for feeding into said store register a number of clock pulses equal to the complement of the number occurring in the time interval corresponding to the phase difference between fundamental frequency signals derived from two multipulse signals received from different stations, means for filling said store register with pulses from said divider with the divider set with a division factor corresponding to the least common multiple frequency for the pair of stations from which signals are being compared and gate means controlled by said store register for gating into said display register further clock controlled pulses for a time period equal to the time period taken to fill the display register.

5. A receiver as claimed in claim 4 and for use with a transmitting system in which a master station radiates normally on the sixth harmonic of a radiation fundamental frequency and three slaves radiate normally on the fifth, eighth and ninth harmonics and wherein in the receiver, oscillators are provided at frequencies 6F, 5F, 8F and 9F locked respectively to the received signals during the normal transmission periods and wherein the clock pulse frequency is 300F and wherein said divider divides the clock frequency by factors of 10, 8 and 6.

6. A receiver as'claimed in claim 5 wherein the clock pulses fed into said display register have a frequency 30F.

7. A receiver as claimed in claim 1 wherein the fine positional information is indicated using a separate indicator for each pattern, each indicator having a pointer which traverses a circular scale, one revolution correspondin to one cycle of phase Qhan e at the least common mul iple frequency of the signa s being compared and wherein each indicator has a mechanical counter driven through reduction gearing to indicate the number of complete revolutions of said pointer and wherein said adjustable divider is set for each comparison at the fundamental frequency so that the digital counter indicates in decimal digits the correct lane count determined by the phase comparison at the fundamental frequency for setting the appropriate mechanical counter.

8. in a receiver for a phase comparison radio navigation system in which phase comparisons between signals normally radiated by spaced transmitters are effected at different multiples of a common fundamental frequency F to provide positional information with respect to fine patterns and in which phase comparisons at the fundamental frequency 1F are made for each pair of stations to provide coarser positional information, the improvement comprising a digital phase comparator to give a digital read-out count for effecting comparisons between signals at a frequency 1F, said digital phase comparator including a clock pulse source, a controllable divider dividing down the clock pulse frequency and a counter counting the divided down frequency pulses, the divider being controlled in accordance with the least common multiples of the normal transmissions from the two stations of each pair whereby the read-out count for each pair of stations is proportional to the time difference corresponding to the phase angle with proportionality factors for the different pairs of stations proportional to the multiples at which the normal signals of the corresponding pairs are compared. 

1. In a receiver for a phase comparison radio navigation system of the kind in which radio frequency signals of three or more different but harmonically related frequencies received from different spaced transmitting stations are compared in phase in pairs at their least common multiple frequencies to provide positiOnal information with respect to fine patterns and in which coarser positional information to identify the lanes of the fine pattern is oBtained by making phase comparisons at the fundamental frequency for each pair of stations; the improvement comprising the provision of digital phase comparison means for the fundamental frequency comparisons, which phase comparison means comprise a clock source, an adjustable divider dividing down the clock pulse frequency, a counter to count pulses from a said divider during a time interval representative of the phase difference between the signals to be compared, and means controlling the divider in accordance with the sequential comparison of signals from different pairs of stations whereby the counter displays counts related to the phase difference, the respective counts, by the controlled adjustment of the divider, being proportional to the respective least common multiple frequencies used for the fine pattern of each pair.
 2. A receiver as claimed in claim 1 wherein each phase difference between the fundamental frequency signals is measured by using the two signals to be compared in phase to gate a clock output at the appropriate frequency.
 3. A receiver as claimed in claim 1 wherein each phase difference between the fundamental frequency signals is measured by measuring the corresponding time difference into a first store using a fixed clock frequency and then counting a fixed clock frequency (not necessarily the same as the aforementioned fixed clock frequency) into a display store from a period determined by the time required to count using a divided clock frequency.
 4. A receiver as claimed in claim 1 wherein said clock pulse source has a frequency which is a multiple of the fundamental frequency, and wherein said adjustable divider for dividing down the frequency of the clock pulse source, said division being by selectable factors in the same ratio to one another as the aforesaid least common multiple frequencies, and wherein there are provided a store register and a display register said store register having a capacity equal to the number of clock pulses in one cycle of the fundamental frequency, means for feeding into said store register a number of clock pulses equal to the complement of the number occurring in the time interval corresponding to the phase difference between fundamental frequency signals derived from two multipulse signals received from different stations, means for filling said store register with pulses from said divider with the divider set with a division factor corresponding to the least common multiple frequency for the pair of stations from which signals are being compared and gate means controlled by said store register for gating into said display register further clock controlled pulses for a time period equal to the time period taken to fill the display register.
 5. A receiver as claimed in claim 4 and for use with a transmitting system in which a master station radiates normally on the sixth harmonic of a radiation fundamental frequency and three slaves radiate normally on the fifth, eighth and ninth harmonics and wherein in the receiver, oscillators are provided at frequencies 6F, 5F, 8F and 9F locked respectively to the received signals during the normal transmission periods and wherein the clock pulse frequency is 300F and wherein said divider divides the clock frequency by factors of 10, 8 and
 6. 6. A receiver as claimed in claim 5 wherein the clock pulses fed into said display register have a frequency 30F.
 7. A receiver as claimed in claim 1 wherein the fine positional information is indicated using a separate indicator for each pattern, each indicator having a pointer which traverses a circular scale, one revolution corresponding to one cycle of phase change at the least common multiple frequency of the signals being compared and wherein each indicator has a mechanical counter driven through reduction gearing to indicate the number of complete revolutions of said pointer and wherein said adjustable divider is set for each comparison at the fundamental frequency so that the digital counter indicates iN decimal digits the correct lane count determined by the phase comparison at the fundamental frequency for setting the appropriate mechanical counter.
 8. In a receiver for a phase comparison radio navigation system in which phase comparisons between signals normally radiated by spaced transmitters are effected at different multiples of a common fundamental frequency F to provide positional information with respect to fine patterns and in which phase comparisons at the fundamental frequency 1F are made for each pair of stations to provide coarser positional information, the improvement comprising a digital phase comparator to give a digital read-out count for effecting comparisons between signals at a frequency 1F, said digital phase comparator including a clock pulse source, a controllable divider dividing down the clock pulse frequency and a counter counting the divided down frequency pulses, the divider being controlled in accordance with the least common multiples of the normal transmissions from the two stations of each pair whereby the read-out count for each pair of stations is proportional to the time difference corresponding to the phase angle with proportionality factors for the different pairs of stations proportional to the multiples at which the normal signals of the corresponding pairs are compared. 